Extrusion screws featuring two or more parallel channels are among the first types of screws used in the trade. These screws, in which all channels within a given screw are of identical configuration, usually of relatively low compression ratio as measured by the ratio of feed to meter channel depth. These types of screws were replaced by the single channel screw which was found to be more effective than the multichannel screw described above. Increasing emphasis was placed on optimization of the single channel screw with the aid of computer programs as these became available. While these developments produced considerable performance improvements they, nevertheless, failed to cope with the variability in materials and process encountered in the trade.
The last decade witnessed the appearance of several unique types of extrusion screws intended to overcome the variability problems described. The most noteworthy of these developments is the barrier screw. While many variations of the basic barrier concept were produced, they all feature a nonparallel minor flight in the transition, i.e. melting, zone whose purpose is to separate the solid from the melt where these two phases coexist. It accomplishes this by allowing the melt to pass over the minor flight which is of a lesser height than the major flight while containing the solid pellets in the original channel. The minor flight is faced out at the calculated point of complete conversion of solid to melt. This condition is only realized when design parameters are satisfied which is rarely the case in practice.